/ Field knowledge · IMC 601.5
A ¾-inch undercut is not a return-air design.
A door undercut is a gap, not a duct. IMC 601.5: size return and transfer openings to the manufacturer, Air Conditioning Contractors of America (ACCA) Manual D, or the design professional. Return taken from a room cannot exceed the supply delivered to that room. Closets under 30 square feet need a 1½-inch undercut, a louvered door, or a transfer grille with at least 30 square inches of net free area — and that closet return can serve only that closet. For other rooms, a ¾-inch undercut at a 32-inch door is only about 24 square inches gross. Higher-airflow rooms usually need a transfer grille or a jump duct. A practical field recommendation is about 1 square inch of transfer opening per 1 cubic foot per minute (cfm) delivered to the room. That 1-to-1 figure is a rule of thumb from Daily Code Talk, not a code minimum.
Educational 2024 International Mechanical Code (IMC) 601.5 from Daily Code Talk. Confirm the adopted edition. Distinct from 601 corridor / prohibited-location returns and 501.4 pressure equalization. This is not a duct design and not a substitute for Manual D or a sealed set. Type I hood replacement air is still IMC 508 — an undercut door is not makeup air for a kitchen hood.
- What the opening is for. Supply air that enters a closed room has to leave. Paths that actually work: a dedicated return duct, a jump duct (a short duct over the door, grille to grille), a transfer grille, a louvered door, or — only when the area is enough — a door undercut. Rank them by how much air you actually delivered, not by what is cheapest to cut.
- 601.5 sizing. Return and transfer openings: manufacturer instructions, ACCA Manual D, or the PE. Do not copy a “typical ¾-inch undercut” onto every door.
- Closets under 30 square feet (code path). 1½-inch door undercut, louvered door, or transfer grille with at least 30 in² net free area. That return serves only that closet.
- Field recommendation for other rooms (not a code minimum). About 1 in² of transfer opening per 1 cfm delivered. A ¾-inch undercut × 32-inch door ≈ 24 in² gross — on the order of 24 cfm on that rule of thumb, before louvers, carpet, and door swing eat the free area. A bedroom getting 120 cfm needs a jump duct or a transfer grille, not a hope.
- Balance. Return from a room cannot exceed the supply to that room. If the only return is a central hall grille, every closed bedroom is a transfer problem.
- Where return cannot be taken (unless a listed exception applies): bathrooms, toilet rooms, kitchens, garages, boiler rooms, furnace rooms, unconditioned attics, crawl space tied to the furnace return, indoor pools unless dehumidified per 403.2.1 item 2 or on a dedicated system, hazardous or insanitary locations, refrigeration rooms, another dwelling unit. Stay 10 feet from an open combustion chamber or draft hood of another appliance in the same room.
On the set: show every return path — dedicated duct, jump duct, transfer grille, or undercut — with net free area where a grille or louver is used. Closet doors under 30 sf get the 1½-inch / 30 in² callout. Room-by-room supply vs return on the schedule is the reviewable proof. If the corridor is the only path back, that is a 601.2 question, not an undercut question.
A ¾-inch undercut on a 150-cfm office
The door slams, the room pressurizes, the diffuser dumps, and the return never sees the air. Draw the jump duct. If rated construction or a smoke barrier is in the way, that opening needs a 607 damper — 512.5 / 607 — not a hole.
Source: Daily Code Talk — IMC 601.5 · tool: Undercut vs jump-duct screen · corridors: 601 · pressure: 501.4 · kitchen: 508.
What to send
Room-by-room supply cfm if you have it, door widths and undercut (or a photo of the bottom of the door), whether any jump ducts or transfer grilles exist, and whether any of those doors sit in a rated or smoke-barrier wall.
If the only return path is a ¾-inch undercut
Send the supply to the room and a photo of the door. Masterbuild will say whether 601.5 still has no opening sized to the air — or the closet still has no 1½-inch / 30 in² path.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. Closet 1½-inch undercut / 30 in² net free area, return not exceeding supply, 10 ft from an open combustion chamber, and sizing to the manufacturer, ACCA Manual D, or the PE are 2024 IMC 601.5 education from Daily Code Talk. The 1 in² per 1 cfm figure and the ¾-inch × 32-inch ≈ 24 in² example are Daily Code Talk field recommendations — not code minimums and not a duct design. This page is not legal advice.